There are many instances where it would be advantageous to be able to display only selected portions of information or written material on an overhead projector. Typed materials displayed on transparencies are sketches, charts, printed matter and the like.
Some attempts have been made to selectively mask portions of the material on a transparency. Porter No. 3,269,261 discloses a masked sheet slidable in a frame to selectively expose material on a transparency. The information in Porter is revealed in only one way or direction. Diehl No. 3,531,193 discloses a transparency holder mounted on rollers and the use of a pair of masking plates slidable in tracks into and out of the projection area. The Diehl device is mainly a transparency holder and moving device that is not portable. Wright No. 3,661,449 discloses a frame defining a window with four slides movably mounted to selectively mask the projector window. The Wright device is in only four quadrants and is not portable. Deary No. 4,688,910 discloses a support frame for a transparency and a cover that selectively presents portions of a transparency in an overhead projector. The Deary device uses an adhesive to attach the support frame to the projector and the cover reveals the information from only one way.